Leakage filter for frequency modulated continuous wave doppler radar system

ABSTRACT

A leakage filter for a frequency modulated continuous wave Doppler radar system is provided. The leakage filter of the present invention replaces, with a single capacitor in each channel and with a receiver blanking switch, the complex leakage filter which has been used in the prior art systems of this general type. The typical prior art filter includes, for example, a capacitor bank in each channel and a complicated switch driver. In the leakage filter of the present invention, a single capacitor in each channel is charged at the beginning of every transition, and the receiver is blanked by the aforesaid receiver blanking switch during each charging interval, as will be described.

LEAKAGE FILTER FOR FREQUENCY MODULATED CONTINUOUS WAVE PrimaryExaminerBenjamin A. Borchelt DOPPLER RADAR SYSTEM Assistant Examiner-G.E. Montone An A. tan t al. [72] Inventor: Robert Slater, Bardonia, N.Y.omey s (harm a e [73] Assignee: The Singer Company, New York, ABSTRACT Aleakage filter for a frequency modulated continuous [22] Filed; Ju|y 1,1 71 wave Doppler radar system is provided. The leakage filter of thepresent invention replaces, with a single [21] PP 158,684 capacitor ineach channel and with a receiver blanking switch, the complex leakagefilter which has been used 52 us. Cl. ..343/8 in the P System of thisgeneral type The p [51] Int. Cl ..G01s 9/44 cal prior art filterincludes for example a capacitor [58] Field of Search n 343/8 7 7 bankin each channel and a complicated switch driver. In the leakage filterof the present invention, a single capacitor in each channel is chargedat the beginning [56] References Cited of every transition, and thereceiver is blanked by the UNITED STATES PATENTS aforesaid receiverblanking switch during each charging interval, as will be described.3,187,330 6/1965 Boles et al. ..343/8 2,909,656 10/1959 Meyer ..343/8 6Claim, 4 Drawing Figures fl AdId/ f/v'rr/hd/fa BY/e/j 1a 14 g j: I 30.aa 54/ ed Law- 7/ I512 Lowhma/a/lbr MZZF FBI/210:; II a I51: I, L"Girl/i1 n, t 3L! ....1

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LEAKAGE FILTER FOR FREQUENCY MODULATED CONTINUOUS WAVE DOPPLER RADARSYSTEM BACKGROUND OF THE INVENTION systems, or in the prior artcontinuous wave radar systems. For example, the prior art continuouswave radar systems have certain disadvantages with regard to transmitterand receiver isolation. The prior art pulse radar systems, on the otherhand, exhibit inherent difficulties in operating to zero altitude.

If transmitter-receiver pulse overlap is used in the prior art pulsetype'of radar system to improve the'low altitude performance thereof,the transmitter leakage increases. The prior art frequency modulationcontinuous wave (FM-CW) type of radar system, on the other hand,assuming that the appropriate modulation frequency, modulation index,and side bands are used, is advantageous in that it can exhibit hightransmitterreceiver isolation. The (F M-CW) Doppler radar system inaddition to exhibiting high transmitter/receiver isolation, can operateto zero altitude, and can measure zero, negative and verticalvelocities. In addition, it is possible to design a (FM-CW) Dopplerradar system which provides absolute altitude measurements.

One of the reasons for the selection of a (FM-CW) Doppler system over apure (CW) radar system is that a much greater transmitter-receiverisolation and discrimination against echoes from nearby objects areafforded by the (FM-CW) Doppler radar system. This obtains because asthe relative delay between the local oscillator signal and the receivedsignal approaches zero, as is the case for leakage signals, the receivedmodulation index (M), and therefore the amplitude of all the Besselsidebands except for J approach to zero. Because of this, the .1 Besselsideband is normally not selected for use in a frequency modulatedcontinuous wave Doppler system.

The (FM-CW) Doppler radar systems usually made use of the J, Besselsideband because of its ability to provide adequate signal-to-noiseratio at both very high and very low altitudes. At the lower altitudes,the decrease of the total signal is balanced by an increase in the Jsideband, resulting in an essentially constant signal-to-noise ratiowhich is independent to altitude. This fact allows a (FM-CW) Dopplerradar system to operate at ground level when using the J sideband. Atthe very high altitudes, where the signal-to-noise ratio is mostcritical, the J, sideband selection results in fairly low modulationlosses.

The J and higher sidebands in the (FM-CW) Doppler radar systems provideconsiderable isolation from transmitter leakage, and from noise causedby reflections from nearby targets. However, components of leakageassociated with the particular sideband, known as sideband leakage,normally are present at the receiver input. Leakage elimination filterssuch as notch filters have been used in the past in an attempt toeliminate this problem, and to reduce sideband leakage to a level whichcan be tolerated at the receiver input.

However, a simple notch filter is usually not sufficient, becausealthough the leakage frequency is known, the switching sidebands due tothe amplitude and phase of the leakage changing for each antenna beamand f.m. frequency combination, appears to the frequency tracker asDoppler echos, and thereby cause false lock-ons and erroneous outputs.In addition, since two or more f.m. frequencies are usually employed inpresent day (FM-CW) Doppler systems, more than one notch filter would berequired in such a system, and this would result in filtering out usefulDoppler information, and would cause tracking errors.

One prior art method of leakage elimination in the (FM-CW) Dopplersystem is to mix the leakage signal and Doppler signal at the f.m.frequency with quadrature components of the signal at the f.m. frequencywhich is coherent with the leakage. The mixing is performed insine-cosine manner in order to preserve the fore-aft sense of theDoppler return. At the output of the mixer, the leakage is at adirect-current level with an amplitude proportional to the amplitude ofthe leakage input and cosine of the phase angle of the leakage relativeto the coherent local oscillator. The sum components are then filteredby a two-pole low pass filter.

However, in the prior art leakage filter systems such as describedabove, and when such a system is used in a typical (FM-CW) Dopplersystem which has, for example, eight combinations of beams and f.m.frequencies, there are eight different leakage levels. Then, in order toavoid the aforementioned problem of switching sidebands being present inthe Doppler pass band to cause erratic tracker operations, the usualprior art system employs a separate capacitor for each of the eightleakage levels. Since there are usually two channels in the system, 16capacitors are therefore required. The capacitors are sequentiallyswitched in synchronism with the beaming and the f.m. frequency, and theleakage is efi'ectively eliminated in the prior art system because thedirect current voltages stored in the various capacitors cancel thedirect current input leakage levels.

However, a disadvantage in the prior art system is the complexity of thedrivers for the switch capacitors, and the number of high quality andbulky capacitors which are required. Also, the prior art system requiresthat the leakage amplitude and phase for a given beam and f.m. frequencycomponent be relatively time invarient. If the orientation of theantenna to the radome should change during a sampling interval, than achange in leakage due to reflections from a radome which has beendegraded due to moisture, oil coating, dirt or the like, is not filtereduntil the prior art leakage filter can recover. This creates a situationin which the leakage signal could enter and saturate the receivercircuitry while the leakage filter was still disabled, so as to produceerratic tracking operations.

The leakage filter of the present invention replaces the capacitor banksand complicated switch drivers of the prior art filter, as describedabove, with a single capacitor in each channel and, as mentioned above,the single switch driver and switching circuit which serves to blank thereceiver for a short period of time, specifically less than 10 percentof its on time, at the beginning of every antenna beam transmission. Theprinciple of operation of the leakage filter of the invention is tocharge a single capacitor in each channel rapidly to the direct currentleakage level at the beginning of every transition. During the trackingtime, and as mentioned above, the receiver is blanked to preventsaturation of the receiver circuitry. Since the time required to chargethe capacitor to virtually its final value is less than percent of theon time, the only adverse effect on the system operation is, forexample, a tolerable 4 percent increase in the Doppler fluctuationerror.

The invention provides, therefore, an improved leakage eliminationfilter for use in a frequency modulation-continuous wave Doppler radarsystem, and which serves to reduce the complexity of the prior artleakage elimination filter presently used in such systems, and whichalso serves to correct certain deficiencies inherent in such prior artsystems. Specifically, the system of the invention is capable ofresponding to leakage level changes due to reflections from a degradedradome, and to filter such reflections, so that the operation of theradar system of the invention is maintained at a high level under allpossible conditions.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a power spectral plot of theinput to the leakage elimination filter of a frequencymodulatedcontinuous wave Doppler radar system;

FIG. 2 is a representation of the leakage input to the leakageelimination filter as it appears as a square wave modulated signal atthe beaming frequency, assuming that there are eight combinations ofbeaming and frequency modulation in the system under consideration,which re-cycles, for example, every 5 seconds;

FIG. 3 is a power spectral plot of the output of the leakage eliminationfilter in the frequency modulationcontinuous wave Doppler radar system;and

FIG. 4 is a functional block diagram of a leakage elimination filterconstructed in accordance with the invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT The input to theleakage elimination filter of a (FM- CW) Doppler radar system isobtained by heterodyning the Doppler-shifted echo with the transmittedsignal and processing one of the Bessel sidebands. A power spectral plotof the input to such a leakage elimination filter is shown in FIG. 1.For a system which processes the Bessel sideband (J and has an arraydeck transmit-receive isolation of about 50 db for example, the leakagepower (L is expected to be about 60 db greater than the minimum Dopplersignal. This necessitates removal of the leakage signal (L before theDoppler return signal can be amplified for frequency tracker operation.

Assuming that there are eight combinations of beatning and frequencymodulating, which recycle every half second, as is usual in many presentday (FM-CW) Doppler radar systems, then the leakage input to the leakageelimination filter included in such a system appears as a square wavemodulated signal, such as shown in FIG. 2. This square wave modulatedsignal is arnplitude modulated by the square wave at the beamingfrequency, for example, 18 Hz, with the carrier at the f.m. frequency. Apower spectral plot of the output of the leakage elimination filter usedin the (FM-CW) radar system is shown in FIG. 3. It will be observed inthe representation of FIG. 3 that the leakage signal L of FIG. 1 hasbeen eliminated, and the fore-aft Doppler shifted signal is preserved.

In the leakage elimination filter system shown in FIG. 4, the inputsignal, including the leakage signal L; as shown in FIG. 1, isintroduced to a pair of balanced heterodyne mixer stages 10 and 12. Aheterodyning signal (sin W t) is applied to the mixer 12 from a localoscillator 11, and a quadrature component (cos W t) is applied to themixer 10 from the local oscillator 11 through a phase shifter 13. Theoutputs from the mixers 10 and 12 are introduced to respective low-passfilters l4 and 16, and through corresponding emitter followers 18 and 20to capacitors 22 and 24.

As mentioned above, the heterodyne mixing of the input signal in thebalanced mixers 10 and 12 is performed in a sine, cosine fashion inorder to preserve the fore-aft sense of the Doppler return signals shownin FIGS. 1 and 3. The leakage signal output of each of the balancedmixers 10 and 12 at the f.m. frequency is a DC level with an amplitudeproportional to the amplitude of the leakage signal and to the cosine ofthe phase angle of the leakage signal relative to the local oscillatorheterodyning signal. The sum components at 2 f.m. are filtered by thetwo-pole low-pass filters I4 and 16. The emitter followers 18 and 20 areused so that the capacitors 20 and 24 will not load the precedingstages.

The capacitors22 and 24 replace the bank of capacitors in the prior artleakage elimination filter system, and the complex logic switch driversused for activating the individual capacitors in each bank in the priorart system are also eliminated. Instead, a single capacitor 22 is usedin one of the channels, and a signal capacitor 24 is used in the other.The capacitor 22 is connected to a switch section SW-l which isconnected to a grounded resistor 26. The capacitor 24 is connected to asecond section of the switch SW-l which is connected to a groundedcapacitor 28.

The capacitor 22 is also connected through a first section of a switchSW-2 to a low pass filter 30, the input terminal of which is connectedto a grounded resistor 32. The capacitor 24 is connected to the secondsection of the switch SW-2 which is connected to a low pass filter 34and to a grounded resistor 36. The switches SW-l and SW-Z may be solidstate switches, and may be driven by an appropriate switch drivercircuit 38 which, in turn, is pulsed by an appropriate switching signalsource, such as a single-shot multivibrator 40. The multivibrator 40 isdriven at a frequency of, for example, 18 Hz to correspond with theswitching modulation on the input signal, as shown in FIG. 2.

The low pass filter 30 is connected to a demodulator circuit 38, and thelow pass filter 34 is connected to a demodulator circuit 40. The outputsfrom the low-pass filters 30 and 34 are demodulated in the demodulators38 and 40 by the in-phase and phase quadrature components (sin W t) and(cos W t) respectively of the signal (f The component (sin W t) isderived from an oscillator 37, and the component (cos W t) is derivedfrom the oscillator through a 90 phase shifting network 39. The outputsfrom the demodulators are applied to a sum amplifier 42, the output ofwhich is passed through a low-pass filter 44 to the receiver.

It will be appreciated that the various components shown in block formin FIG. 4 are of usual form, and need not be described in circuitdetail, insofar as a clear understanding of the present invention isconcerned.

As explained above, the leakage elimination filter system of FIG. 4 usesa single capacitor in each channel to replace the capacitor bank, andcomplicated switch drivers of the prior art system. The principle ofoperation of the filter system of FIG. 4 is to charge rapidly thecapacitors 22 and 24 to a direct current level corresponding to theamplitude of the leakage signal in FIG. 1, and this is carried out atthe beginning of each transition, as the multibeam multi-frequency radarsystem is switched from one mode to another. During the charging time ofthe capacitors, the switch driver 38 opens the switch SW-2 and closesthe switch SC-l, so that the capacitors 22 and 24 may receive thecharge, and so that the receiver may be blanked during the chargingintervals.

When the switch SW-l is closed and the switch SW-2 is open, thecapacitors 22 and 24 rapidly charge to virtually 100 percent of thedirect current leakage levels. This occurs during a blanking time of,for example, 2.2 milliseconds which effects a greater than 60 Db leakagerejection. After the blanking period, the switch SW-l is opened and theswitch SW-2 is closed, and the Doppler return signals, shown in FIG. 3,with the direct current leakage level eliminated are then demodulated inthe demodulator circuits 38 and 40 as frequency modulations about thetracker frequency f,,. The signal output from the low pass filter 44then has the form shown in FIG. 3 with the Doppler return appearing assidebands of the tracker frequency f and with the leakage eliminated. Asmentioned above, the reason two channels are used in the filter systemof FIG. 4 is to preserve the fore and aft sideband components of theDoppler return signal, as shown in FIG. 3.

The invention provides, therefore, an improved leakage eliminationfilter system for use in a (F M-CW) Doppler radar system, and whichfinds particular utility in a multi-beam multi-frequency system of theaforesaid type. The filter system of the invention is advantageous inthat it serves to eliminate the leakage level signals in such a radarsystem, as described, and of achieving its purpose in a relativelysimple manner, and with a minimum of electronic components.

While a particular embodiment of the invention has been shown anddescribed, modifications may be made. It is intended to cover allmodifications which come within the spirit and scope of the invention inthe following claims.

What is claimed is:

1. A leakage elimination filter for use in a multibeam switch frequencymodulation-continuous wave Doppler Radar System, with filter respondingto at least one of the Bessel sidebands of a Doppler-shifted echo signalreceived in such a system, and a filter also responding to a leakageinput having the frequency modulation frequency of the system and squarewave amplitude modulated at the switching rate of the system, the filterincluding:

a first channel comprising a heterodyne mixer stage,

a first capacitor element coupled to the output of said mixer stage, anoutput circuit, means including a first switching unit forintermittently disconnecting the capacitor element from said outputcircult, and means including a second switching unit for intermittentlyconnecting said capacitor element to a point of reference potential topermit said capacitor element to be charged by the output of theheterodyne mixer stage;

local oscillator means for introducing a signal to said heterodyne mixerstage having a frequency corresponding to the frequency modulationfrequency of the system to cause said heterodyne mixer stage to producea direct current charging voltage for said capacitor elementcorresponding to the input leakage levels;

a switch drive circuit coupled to said first and second switching unitsand synchronized with the aforesaid switching rate of the'systemactuating said switching unit for each amplitude transition of theaforesaid input, so that said first switching unit is intermittentlyclosed to permit said capacitor element to become charged while saidsecond switching unit is open, and so said second switching unit may beclosed in alternate intervals to connected said output circuit to saidcapacitor element while said first switching unit is open.

2. The leakage elimination filter defined in claim 1 in which saidoutput circuit includes a frequency modulation demodulating circuit, andin which said filter includes further oscillator means for generating amodulating signal for said demodulating circuit.

3. The leakage elimination filter defined in claim 1 and which includesa second channel comprising a second heterodyne mixer stage, a secondcapacitor element coupled to the output of said second mixer stage, asecond output circuit, means including a first switching unit forintermittently disconnecting said second capacitor element from saidoutput circuit, and means including a second switching unit forintermittently connecting said second capacitor element to a point ofreference potential; and in which said local oscillator means introducessaid signal to a first heterodyne mixer stage in phase with saidfrequency modulation frequency signal, and in which said localoscillator introduces said signals to said second heterodyne mixer stagephase quadrature relationship with the frequency modulation frequencysignal.

4. The leakage elimination filter defined in claim 3 in which said firstswitches in the first and second chan nels, and said second switches inthe first and second channels are actuated together.

5. The leakage elimination filter defined in claim 3 in which said firstoutput circuit in the first channel comprises a first frequencymodulation demodulating circuit, and said second output circuit in saidsecond channel comprises a second frequency modulation demodulatingcircuit; and which includes a further oscillator for introducing ademodulating signal to said first demodulating circuit and forintroducing a phase guadrature demodulating signal to said seconddemodulating circuit.

6. The leakage elimination filter defined in claim 5 and which includessum amplifier means coupled to said first and second demodulatingcircuits and respon sive to the outputs thereof to produce a commonoutput from said leakage elimination filter.

1. A leakage elimination filter for use in a multi-beam switch frequencymodulation-continuous wave Doppler Radar System, with filter respondingto at least one of the Bessel sidebands of a Doppler-shifted echo signalreceived in such a system, and a filter also responding to a leakageinput having the frequency modulation frequency of the system and squarewave amplitude modulated at the switching rate of the system, the filterincluding: a first channel comprising a heterodyne mixer stage, a firstcapacitor element coupled to the output of said mixer stage, an outputcircuit, means including a first switching unit for intermittentlydisconnecting the capacitor element from said output circuit, and meansincluding a second switching unit for intermittently connecting saidcapacitor element to a point of reference potential to permit saidcapacitor element to be charged by the output of the heterodyne mixerstage; local oscillator means for introducing a signal to saidheterodyne mixer stage having a frequency corresponding to the frequencymodulation frequency of the system to cause said heterodyne mixer stageto produce a direct current charging voltage for said capacitor elementcorresponding to the input leakage levels; a switch drive circuitcoupled to said first and second switching units and synchronized withthe aforesaid switching rate of the system actuating said switching unitfor each amplitude transition of the aforesaid input, so that said firstswitching unit is intermittently closed to permit said capacitor elementto become charged while said second switching unit is open, and so saidsecond switching unit may be closed in alternate intervals to connectedsaid output circuit to said capacitor element while said first switchingunit is open.
 2. The leakage elimination filter defined in claim 1 inwhich said output circuit includes a frequency modulation demodulatingcircuit, and in which said filter includes further oscillator means forgenerating a modulating signal for said demodulating circuit.
 3. Theleakage elimination filter defined in claim 1 and which includes asecond channel comprising a second heterodyne mixer stage, a secondcapacitor element coupled to the output of said second mixer stage, asecond output circuit, means including a first switching unit forintermittently disconnecting said second capacitor element from saiDoutput circuit, and means including a second switching unit forintermittently connecting said second capacitor element to a point ofreference potential; and in which said local oscillator means introducessaid signal to a first heterodyne mixer stage in phase with saidfrequency modulation frequency signal, and in which said localoscillator introduces said signals to said second heterodyne mixer stagephase quadrature relationship with the frequency modulation frequencysignal.
 4. The leakage elimination filter defined in claim 3 in whichsaid first switches in the first and second channels, and said secondswitches in the first and second channels are actuated together.
 5. Theleakage elimination filter defined in claim 3 in which said first outputcircuit in the first channel comprises a first frequency modulationdemodulating circuit, and said second output circuit in said secondchannel comprises a second frequency modulation demodulating circuit;and which includes a further oscillator for introducing a demodulatingsignal to said first demodulating circuit and for introducing a phaseguadrature demodulating signal to said second demodulating circuit. 6.The leakage elimination filter defined in claim 5 and which includes sumamplifier means coupled to said first and second demodulating circuitsand responsive to the outputs thereof to produce a common output fromsaid leakage elimination filter.